Introductory Chapter: Phase Change Material as Energy Storage Substance

1. Introduction

The economic success and technical competitiveness of a country are significantly influenced by its access to energy. World energy outlook [1] reported that the total energy demand for the countries like India and China is expected to increase from 55% in 2010 to 65% in 2035. Due to factors such as population growth, fast urbanisation, and economic development, demand for easily accessible energy sources like fossil fuels is rising at an almost exponential rate. Despite the development of resources and the expansion of energy supplies, enough energy has not been produced to fulfil the rising demand. Additionally, the demand for energy in various sectors like residential and commercial, industrial, and utility fluctuates on a time basis. In order to meet such variation, it is essential to look for suitable means of storage of energy.

The storage of energy offers an alternate approach to meeting peak demand for electricity. The most expensive and challenging times to provide are during peak hours. Therefore, energy storage systems offer a tremendous amount of untapped potential to improve the efficiency of equipment that converts energy. It also has the potential to be utilised for enabling large-scale fuel replacements in economies all around the world. Energy storage results in cutting down the energy wastage. This is a cheaper, easier, and faster way to address several energy issues. In a nutshell, energy storage not only acts as a link between demand and availability but also boosts the system’s overall performance and dependability. Consequently, the development of energy storage systems that are efficient and low-cost is an equally vital subject of study as the development of new sources of energy.

2. Energy storage systems

The act of temporarily ‘storing’ energy for the purpose of using it at a later time is referred to as energy storage (ES). Power and energy services rely on ES devices, which take in excess energy and hold it for later use. Energy may be stored in a variety of forms, including mechanical, electrical, chemical, and thermal energy. The ES systems are generally categorised by output: electricity and thermal.

2.1 Electricity storage systems

Electricity storage systems include hydropower, compressed air energy, flywheels, batteries, supercapacitors, chemical-hydrogen storage, etc.

• Pumped Storage Hydropower (PSH) systems take use of fluctuations in elevation to accumulate off-peak electricity for use at a later time. During times of lesser demand, water is moved by pump from one reservoir to another reservoir located at a higher elevation. After that, the water is released to begin its descent back to the lower reservoir, which results in the generation of electricity in a manner analogous to that of a simple hydropower plant.

• Compressed Air Energy Storage (CAES) systems utilise energy generated during non-peak hours in order to compress air. This compressed air is then stored in underground storage tanks. Later on, this air is sent out into the combustor of a gas turbine, which helps create energy during peak demand times.

• Flywheels that spin at very high rates can be used to store electricity in the form of rotational energy. The rotor of the flywheel is slowed down to release this stored energy in small bursts.

• Batteries facilitate the flow of electrons by using chemical processes that take place inside two or more electrochemical cells. Sodium sulphur batteries, lead-acid batteries, and lithium-based batteries are some examples.

• Energy is stored in supercapacitors by enormous electrostatic fields that are created between two conducting plates that are very slightly separated from one another. This technique allows for the rapid storage and release of electrical energy, which may result in the production of brief surges of power.

• Chemical hydrogen storage is a method of storing electricity where hydrogen is used as the energy carrier. After going through a series of transformations, electrical energy is stored until it is needed.

2.2 Thermal storage systems

The requirement of a thermal storage system arises in many applications. These include harnessing intermittent sources of energy like solar, wind, etc. and heat recovery systems. Thus, developing efficient and reliable thermal storage devices is an important requirement for both conventional and non-conventional thermal energy systems. Thermal storage system includes sensible heat storage (i.e. solid media storage, hot- and cold-water storage, underground thermal storage), latent heat storage (i.e., ice storage, molten salts), and thermochemical storage.

2.2.1 Sensible heat storage

Practical Heat Storage (SHS) is the process of storing heat by increasing the surrounding temperature. Increasing the temperature of a substance, whether it be a solid or a liquid, serves as a means of storing thermal energy in this system. In SHS systems, the amount of stored heat depends on the storage material, the specific heat of the substance, and the change in temperature. The need for a high volume of material is one of the most significant drawbacks of SHS, particularly when the acceptable temperature range is narrow.

2.2.2 Latent heat storage

Latent heat storage units (LHSUs) are used to store energy in a substance that changes phase when heat is added or removed. When a medium goes from one state to another—solid, liquid, or gas—it is said to have undergone a phase change. Whether energy is being absorbed or discharged determines the direction of this transition (i.e., liquid to solid or solid to liquid).

In comparison to SHS systems, LHSU is a more appealing option. This is due to its high energy density and very narrow temperature range. LHSU also offers consistent energy storage and delivery in a compact design. In the case of water, for instance, the latent heat of fusion is eighty times that of the sensible heat. As a result, at a constant temperature of 0 degrees Celsius, latent heat-based storage systems that utilise water may extract 80 times as much energy as a sensible heat-based storage system. This implies that the quantity of material for a specific amount of energy may be drastically reduced in both weight and size. It is to be noted that the energy storage capacity of a LHSU is generally 5–10 times higher than that of an SHS system [2, 3, 4, 5].

Among the several energy storage technologies discussed above, some technologies have undergone significant research and demonstrations and have matured for commercialization. These include pumped storage technologies, sensible heating technologies etc. Some technologies like thermochemical, supercapacitor, flywheel (high speed), flow batteries, etc., are still in the preliminary stage of research. Technologies like latent heat storage units (LHSU), flywheel (low speed), compressed air energy storage, etc., however fall in the category where there need more research and demonstrations before they can be brought to commercialization. More work has to be done on these systems before they can reach their full potential.

3. Phase change materials

In an LHSU, energy is kept in the latent heat storage material. The term “Phase Change Materials” refers to the types of substances that may store latent heat (PCMs). Telkes and Raymond [6] were among the first to pioneer the research of PCMs. However, before the energy crises of the 1970s and 1980s, nobody paid any attention to it, when there was intensive research on the use of PCMs in many applications, particularly for solar heating systems. There have been several PCMs found and investigated extensively for their application in LHSU during the last four decades, and these PCMs span a wide range of melting/freezing points. A classification of these PCMs is given in Figure 1.

Figure 1.

Classification of PCMs [5].

Many factors, including thermodynamic, chemical, kinetic, and economic ones, go into deciding which PCM to use. To be considered as PCMs, a substance must meet the following criteria:

• When choosing a PCM for the first time, it’s important to make sure its melting point falls within the system’s optimal working temperature range.

• To alleviate the containment issue, the PCM should have a low vapour pressure at an operational temperature, a modest volume expansion during the phase transition, and a high latent heat.

• Insignificant super cooling as the super cooling will reduce the heat storage capacity of PCMs.

• High thermal conductivity to enhance the high heat transfer rates and also high specific heat.

• A high nucleation rate is also required for kinetic properties to reduce supercooling and a high rate of crystal formation.

• Chemicals with non-toxic, non-explosive, and non-flammable properties are included in the category of chemical qualities. The capacity to completely reverse the melting/solidification cycle, chemical stability, the absence of any chemical breakdown, and corrosion resistance to building materials are all important properties.

• Abundant supply at minimum cost.

At least three components are needed for an LHSU: a PCM with an appropriate melting point, a heat exchange surface, and a PCM-compatible container. Thus, Phase Change Materials (PCMs), Containers, and Heat Exchangers must be understood to build an efficient LHSU.

Paraffin and nonparaffin organics are organic substances. Organic compounds offer noncorrosiveness, less cooling, chemical stability and thermal stability, harmonious melting ability, and suitability with common construction materials. Because of the broad temperature ranges across which it may melt or solidify and its large latent heat capacity, paraffin is a frequently used PCM for heat storage. During the process of solidification, they do not cause any subcooling effects and experience just a little volume change during the phase transition. They are stable, nontoxic, and noncorrosive over time. Lauric, myristic, palmitic, and stearic acids are used as PCMs in non-paraffin organics. Predicting melting and solidification behaviour and eliminating subcooling effects are their advantages. Organic compounds are combustible, have poor heat conductivity, and low phasechange enthalpy. Inorganic substances include salt hydrates, metals, alloys, and salts. Salt hydrates were explored, including sodium sulphate decahydrate (Glauber’s salt), calcium chloride hexahydrate, etc. Inorganic compounds have excellent thermal conductivity and volumetric latent heat storage, sometimes double that of organic compounds. Most salt hydrates have problems with super cooling, phase segregation, corrosion, and thermal stability.

3.1 Drawbacks of PCM

PCM with lower thermal conductivity has a major effect on the efficiency of the device. Relatively large temperature reductions are observed during the energy withdrawal or retrieval process when conductivity values are reduced. Because of this, PCM melting and solidification rate has been slower than predicted, and the deployment of large-scale LHTS units has proved unsuccessful. Incomplete melting/solidification and a wide temperature disparity inside the PCM are common outcomes of this scenario, and they can contribute to the eventual failure of the material and overheating of the system. Therefore, it is even more crucial to develop certain thermal improvement measures to improve the thermal performance of LHSU.

3.2 Thermal enhancement techniques

The most studied thermal performance enhancement techniques are as follows: -

1. Use of extended surface/Fins

2. Micro Encapsulation

3. High thermal conductivity Nano particles added to the PCM

4. Add PCM into the Porous metallic foams

3.2.1 Use of extended surface/fins

Fins implanted in the PCM are the most widely used of all of these approaches. Fin configurations in the PCM-LHTES are classed as longitudinal, circular/annular, plate, and annular/pin fin. The selection was influenced by the benefits of a larger heat-transfer surface, simplicity, ease of production, and cheap cost of construction. Fins are utilised in thermal systems to provide greater heat transmission surface. Various academics have looked into the significance of different fin layouts in improving performance in the LHSU.

3.2.2 Micro encapsulation

Microencapsulated PCMs (MPCMs) may also improve heat transmission between the source and PCM. Microencapsulated PCMs are micro-sized PCMs that are either liquid or solid at the core and are surrounded by a solid shell or wall. There is a vast range of possible materials that may be used to construct the shell, including synthetic and natural polymers. Such MPCMs can be accomplished using a variety of chemical (e.g., coacervation, complex coacervation, interfacial methods), mechanical, or physical processes (e.g. spray drying method). It is to be anticipated that the thermal performance of MPCMs will outperform that of PCMs that are used traditionally. The heat transmission rate is greater for small PCM particles due to larger heat transfer area per unit volume. There are also advantages to using MPCMs, such as the ability to withstand changes in volume during a phase transition and less PCM reactivity with the container material.

3.2.3 High thermal conductivity nano particles added to the PCM

In spite of the fact that the impact is heavily dependent on the dispersion of the nanomaterial additions in PCM, carbon nanomaterial additives can enhance the PCM’s thermal conductivity. During this process, the nanomaterial is distributed evenly throughout the material, resulting in uniform composite phase change material (CPCM). For the purpose of ensuring that the effectively enhanced thermal conductivity while simultaneously avoiding chemical reactions, the additives need to have a high thermal conductivity and be chemically stable. Metal oxides, carbon nanomaterials such as single- and multi-walled carbon nanotubes, metal nanoparticles, graphite and graphene are commonly used as additives to increase the thermal conductivity of PCM.

3.2.4 Add PCM into the porous metallic foams

Metal matrices constructed of aluminium, copper, and other metals, as well as naturally occurring porous materials like graphite, can be used to create porous structures. Incorporating high thermal conducting material with a PCM storage system improves heat transfer (latent heat phase). The capillary forces and surface tension in the combination of PCM and ceramic structure keep the molten PCM contained and stabilised inside the micro-porosity of the structure, enabling the use of direct contact heat exchangers. Graphite’s higher thermal conductivity, lower density, and chemical resistance make it a promising heat transfer enhancer. The fraction of the composite that is composed of expanded graphite has a significant impact on the effective thermal conductivity of the material, as seen in Figure 2. There should be limits placed on the graphite content since increasing it will drive up production costs and reduce the material’s volume storage capacity.

Figure 2.

Obtained thermal conductivity of PCMs with MgCl₂·6H₂O/EG for different mass fractions of expanded graphite (EG) [7].

3.2.4.1 Closure

Due to the fact that no one material exists with all the desirable characteristics of an ideal thermal storage media, it is necessary to make do with the material at hand and, if necessary, compensate for its subpar physical characteristics through innovative system design. Thus, selecting a PCM is challenging for researchers. In addition to its usage in the energy storage system, PCMs have a wide range of other potential uses in a variety of fields. A number of researchers have reportedly completed a review on the technical particulars of various PCMs and the applications for which they are used. Table 1 is a summary of some of the more important uses of PCMs that have been cited by a variety of researchers as having undergone rigorous review.

Applications	References
Latent heat storage for solar heating systems	[8, 9, 10]
Waste heat recovery	[11, 12, 13, 14]
Building air conditioning	[15, 16, 17, 18]
Building envelope	[19, 20, 21]
Refrigeration system	[22, 23]
Electronics cooling	[24, 25, 26]
Textiles	[27, 28]
Foods	[29, 30]

Table 1.

Applications of phase change materials (PCMs).

Inadequate LHSU performance is caused by the poor heat conductivity of commercially available PCMs, variation in thermo-physical properties after number of thermal cycles, phase separation, volume expansion and high cost [5]. There is a need to devise mechanisms for augmentation in thermal performance of LHSU under these circumstances. Thus, for development of state-of-the-art LHSU, comprehensive investigations are required incorporating the effect of all operating, geometric and design parameters of interest. The results of these analyses can be used as a starting point for further process optimization and the development of design principles.